Artificial seasoning of timber

ABSTRACT

The invention relates to an improved method of preventing bowing, cupping twisting or springing of boards or planks of lumber. During the latter portion of the timber drying schedule i.e. after the lumber has reached its fibre saturation point, the boards or planks are passed between spaced pairs of rollers arranged to hold the lumber in a fixed plane. The lumber is therefore held in this plane while it is setting. Preferably the drying schedule includes intermittent exposure of the lumber to microwave radiation in an electronic kiln dryer.

United States Patent 1191 Pless ARTIFICIAL SEASONING OF TIMBER [76]Inventor: John Henry Pless, 48 Kinnerton St.,

London, S.W.l, England [22] Filed: Sept. 8, 1969 [21] Appl. No.: 855,880

[30] Foreign Application Priority Data 9 Sept. 6, l968 Great Britain42,609/68 [52] US. Cl. 34/1 [51] Int. Cl B01k 5/00 [58] Field of Search34/1 [56] References Cited UNITED STATES "PATENTS 3,474,544 10/1969Holden, Jr. et a]. 34/1 11 3,744,147 July 10,; 1973 PrimaryExaminer-Charles Sukalo Attorney-Francis D. Stephens and Hugo Huettig,Jr;

[57] I ABSTRACT The invention relates to an improved method ofpreventing bowing, cupping twisting or springing of boards or planks oflumber. During the latter portion of the timber drying schedule i.e.after the lumber has reached its fibre saturation point, the boards orplanks are passed between spaced pairs of rollers arranged to hold thelumber in a fixed plane. The lumber is there fore held in this planewhile it is setting. Preferably the drying schedule includesintermittent exposure of the lumber to microwave radiation in anelectronic kiln I dryer.

6 Claims, 2 Drawing Figures Pmmimu I 3.144.141

Q I noenlor a IV Allorney ARTIFICIAL SEASONING F TIMBER It is well-knownthat one of the problems involved in drying lumber (i.e. boards orplanks cut from logs of timber) is that differential rates of dryingthroughout the volume of the lumber can cause cupping, bowing, twistingand springing of the lumber. In conventional kilning processes fordrying the lumber, these types of deformation are curbed by placing theboards or planks under heavy weights in the kiln. This means that thelumber at the bottom of a stack is subjected to a higher pressure thanlumber at the top of the stack. While this may be satisfactory for thelumber at the bottom, (a normal practice being to put the best pieces oflumber at the bottom of the stack), the process is obviously notentirely satisfactory and it is found that the deformations are notcompletely removed from lumber at the top of the stack. Moreover, theprocess is not applicable to more recent high speed kiln dryingprocesses where lumber is continuously moved along a conveyer throughhigh temperature drying installations.

According to the present invention a method of drying boards or planksof lumber in which the lumber is heated to reduce the moisture contentto the desired final level, includes: passing the lumber between pairsof rollers arranged to hold the lumber in a fixed plane for at leastthat part of the drying process after the fibre saturation point hasbeen reached, the unsupported length of lumber between adjacent pairs ofrollers being not more than five feet, each pair of the rollers beingeither fixed or resiliently biased toward one an other and the spacingbetween the rollers in each pair, or the biasing force urging therollers together, being such that substantially no compressive force isapplied to correctly sawn lumber moving between the rollers, whereby thelumber sets in a straightened condition and cupping, bowing, twisting orspringing of the lumber is prevented. I Preferably the rollers aresituated within an electronic kiln drying tunnel but they may also bepositioned toward the end of a high temperature convection heat dryingprocess where lumber is subjected to alternating treatment with dry airat a temperature above 100 C and with wet steam. The rollers are made ofa material which is harder than the lumber passing between them, forexample cement, stone, glass, porce- -lain or metal. The spacing betweenthe rollers in each pair is determined by the standard thickness of thelumber being fed through the process, and the number of pairs of rollerswill depend on the speed at which the lumber is moving and the distancebetween each pair of rollers. The width of the rollers depends onthenumber of pieces of lumber being fed through side by side and the widthof each piece.

If the rollers are fixed, the rollers of each pair are spaced apart justsufficiently for the standard size lumber to pass between them and henceany oversize lumber is compressed between the rollers. If the rollersare resiliently biased toward one another, the pressure between therollers is kept low enough not to compress the standard size lumber.Excessive pressure must be avoided at low moisture content levels sincethe lumber has lost some of its elasticity and is setting, and excessivepressure will therefore cause splitting.

The rollers are preferably driven and thereby act to move the lumberduring the drying process. The movement may include a forward andbackward motion in the electronic kiln or between steam and dry airchambers in a convection process. The distance between the rollers ineach pair must then be such that sufficient frictional contact'betweenthe rollers and the lumber occurs to prevent slippage.

One example of the invention will now be described with reference to theaccompanying drawings in which FIG. 1 is a side elevational view of aelectronic kiln drying process embodying the invention and FIG. 2 is aperspective view of a detail of the electronic kiln shown in FIG. 1.

Referring to these Figures, lumber is fed into a kiln drying tunnel 1 ona conveyer 2. The initial portion of the tunnel comprises a pre-heatingchamber 9 in which lumber is dried approximately to its fibre saturationpoint (around 25 percent moisture content). This chamber is filled withrelatively low power microwave radiation to permit dielectric heating ofthe lumber over a relatively long period. Thereafter the tunnel is splitinto much shorter sections comprising alternate high power microwavedrying chambers 3 and control sections 4. Each microwave chamber has itsown waveguide 5 and microwave generator 6. Microwave energy is launchedthrough the roof of each chamber to produce a rapid burst of dielectricheating as the lumber moves through the chamber. The power consumptionof electrical energy per cu. ft. varies according to the density andthickness of the lumber and lies between 2 and 15 kilowatts per cubicfoot. As much energy as possible is fed into each chamber withoutdamaging the timber. The maximum permitted energy concentration ispreviously calibrated using lumber having known density, thickness andmoisture contents. Above the permitted maximum and steam pressure withinthe lumber becomes too great and the wood ruptures.

For a 1 inch soft wood having a moisture content of 20 percent themaximum power concentration is 12 kilowatts per cu. ft. for 1 minute. Onthe other hand, for 3 inch hardwood with a moisture content of percentthe maximum power concentration is only 2 to 4 kilowatts per cu. ft.This is because softwood is more permeable to the escaping steam andthere is therefore less build-up of pressure. However, sustainedexposure to the permitted maximum power concentrations would again causerupture of the wood, and it is therefore essential that control sections4 lie between the microwave chambers, the control sections beingpreferably three times as long as the microwave chambers.

Within these control sections are the control rollers 7. If, forexample, 1 inch boards of Perana pipe are being fed through the kiln,the maximum distance between adjacent pairs of rollers should not exceedthree feet. On the other hand, for 2 inch boards of Hemlock, theadjacent roller pairs may be up to five feet apart. The purpose of therollers is to hold the boards or planks in a fixed plane during thedrying process. Before entering the kiln, the boards have already beendried to their fibre saturation point and the control rollers aretherefore only acting on the boards or planks in the latter stages ofthe drying schedule. It has been found that these rollers almostcompletely eliminate any cupping, bowing, twisting or springing of thelumber after it is fed out of the kiln.

Typically, a three foot long reinforced steel tubing roller may have adiameter of 2% inches. Such a roller would be able to withstandpressures of 1,000 tons per square inch and hence any oversize lumberwill be squashed between the rollers during its passage through thekiln. Normally, however, the pressure on the lumher is fairly low, therollers merely acting to hold the lumber straight while it sets.

The control chambers may also include watersprays for spraying thesurface of the lumber to control the rate of evaporation. The surface isthus kept moist until the centre of the lumber dries out.

A steam pipe 8 runs the complete length of the tunnel and maintains ahigh ambient temperature. Since the drying is effected by dielectricheating which drives steam out from the centre of the lumber, the highambient temperature helps to minimise the amount of power consumed inreducing the moisture content to a given level.

l claim:

1. A method of drying wooden boards in which the wood is heated toreduce the moisture content to the desired final level, including:passing the boards between fixed pairs of rollers arranged to hold theboards in a fixed plane for at least that part of the drying processafter the fibre saturation point of the wood has been reached, theunsupported length of boards between adjacent pairs of rollers being notmore than five feet, and the spacing between the rollers in each pairbeing such that substantially no compressive force is applied to thecorrectly sawn boards moving between the rollers, whereby the boards setin a straightened condition and cupping, bowing twisting or springing ofthe boards is substantially prevented.

2. The method of claim 11 in which the boards are altemately passedthrough heating section between adja cent pairs of rollers to exposesaid boards to heat and through control sections in which said rollershold said boards and said boards are exposed to controlled dryingconditions.

3. The method of claim 2 in which said boards are exposed for a longertime in said control sections than in said heating sections.

4. The method of claim 3 including the step of spraying the boards withwater as they pass through said control sections.

5. The method of claim 4 including the step of maintaining temperaturesof C and a relative humidity of at least 35 percent in said controlsections.

6. A method of drying boards in which the boards are heated to reducethe moisture content to the desired final level, including: the step ofpassing the boards between pairs of rollers arranged to hold the boardsin a fixed plane during that part of the drying process after the fibresaturation point of the boards has been reached, the unsupported lengthof boards between adjacent pairs of rollers being not more than 5 feet,each pair of the rollers being resiliently biased toward one another,the biasing force urging the rollers together being such thatsubstantially no compressive force is applied to the correctly sawnboards moving between the rollers, whereby the boards set in astraightened condition and cupping, bowing, twisting or springing of thelumber is substantially prevented.

1. A method of drying wooden boards in which the wood is heated toreduce the moisture content to the desired final level, including:passing the boards between fixed pairs of rollers arranged to hold theboards in a fixed plane for at least that part of the drying processafter the fibre saturation point of the wood has been reached, theunsupported length of boards between adjacent pairs of rollers being notmore than five feet, and the spacing between the rollers in each pairbeing such that substantially no compressive force is applied to thecorrectly sawn boards moving between the rollers, whereby the boards setin a straightened condition and cupping, bowing twisting or springing ofthe boards is substantially prevented.
 2. The method of claim 1 in whichthe boards are alternately passed through heating sections betweenadjacent pairs of rollers to expose said boards to heat and throughcontrol sections in which said rollers hold said boards and said boardsare exposed to controlled drying conditions.
 3. The method of claim 2 inwhich said boards are exposed for a longer time in said control sectionsthan in said heating sections.
 4. The method of claim 3 including thestep of spraying the boards with water as they pass through said controlsections.
 5. The method of claim 4 including the step of maintainingtemperatures of 120* C and a relative humidity of at least 35 percent insaid control sections.
 6. A method of drying boards in which the boardsare heated to reduce the moisture content to the desired final level,including: the step of passing the boards between pairs of rollersarranged to hold the boards in a fixed plane during that part of thedrying process after the fibre saturation point of the boards has beenreached, the unsupported length of boards between adjacent pairs ofrollers being not more than 5 feet, each pair of the rollers beingresiliently biased toward one another, the biasing force urging therollers together being such that substantially no compressive force isapplied to the correctly sawn boards moving between the rollers, wherebythe boards set in a straightened condition and cupping, bowing, twistingor springing of the lumber is substantially prevented.